I don't believe they would determine or even expect it is following an algorithm.
For example on HN my password could be 1YCju7i8!
It isn't, but let's pretend it is.
That is a counter at the front, YC is from the website address, j from my user name, and u7i8! It's the unique key. I use the same key everywhere so it's easy to remember.
I don't think a hacker would figure it out.